Feedback regulated power supplies for electronic equipment receive unprocessed and unregulated (uncontrolled) direct voltage from a source of direct voltage such as a motor driven generator, batteries floating on the output of a generator or from the alternating current (AC) mains by way of a rectifier and filter capacitor arrangement. Such uncontrolled direct voltage (often known as DC voltage) may vary widely in amplitude, and may include transient fluctuations. Modern electronic equipment often requires energizing voltage which is closely controlled in magnitude and free of significant perturbations. Power supply regulators accept uncontrolled direct voltage and process the direct voltage by means of feedback regulation to generate regulated direct voltage for application to the equipment to be powered.
Simple regulators include a variable impedance element or active device serially connected between the source of direct voltage or the voltage to be regulated and the equipment to be supplied with regulated voltage. The series impedance of the active device is controlled in a feedback manner to form what amounts to a voltage divider having a controllable voltage division ratio which adapts in order to maintain the tap voltage constant or regulated. This well known type of regulator may not be satisfactory for some applications because of its relatively low efficiency. For those cases in which highest efficiency is desired, or when a regulated output voltage is desired which is greater than the voltage supplied to the regulator, a switching regulator may be used. In general, switching regulators include a high speed solid-state switch serially connected with an inductor. The operation of the switch is controlled in such a fashion as to generate a varying current in the inductor. A capacitor integrates the current from the inductor to form the regulated output voltage. Such switching regulators have relatively high efficiency because their principal elements are relatively lossless reactances and switches. As mentioned, switching regulators can be arranged to either increase or decrease the applied input voltage.
The switching rates of switching regulators at the current state of the art are in the kilohertz range, and extend into the hundreds of kilohertz for some applications. At such high frequencies, the reactances of the wires leading from the source of uncontrolled voltage to the regulator can perturb operation of the regulator. It is common practice to connect a capacitor across the input terminals of the regulator to provide a known low impedance source of current at the input to the regulator. In order to prevent radiation of signal from the input conductors of the regulator at the switching frequency and at harmonics of the switching frequency, the input capacitor is often part of a low pass filter arranged to reduce switching transients on the input conductors. This low pass filter also acts on the uncontrolled direct current flowing in response to the uncontrolled direct voltage from the source of uncontrollable direct voltage to the regulator to reduce the magnitude of transients.
There are many applications in which more than one switching regulator and its input capacitor are connected to a single source of uncontrolled direct voltage. For example, a ship or an airplane may have a single engine-driven generator producing unregulated direct current which is routed by way of cables to various locations, at each of which a switching regulator and its inpt capacitor is connected to the cables for being energized therefrom. In order to be able to deenergize an equipment powered by a switching regulator in such a context, a switch is connected between the source of uncontrolled direct voltage and the input terminals of the switching regulator. Closing the switch allows uncontrolled direct current to be applied to the input capacitor and to the switching regulator, which thereafter operates to produce regulated direct voltage with which to energize its associated equipment. Opening the switch deactivates the switching regulator and the equipment. In the context of an airplane or a ship, it should be understood that the switch may be remotely operated, i.e., a relay. At initial turn-on of a switching regulator as by closing a switch connected between the input terminals of the regulator and a source of direct voltage, a large inrush current surge may flow through the switch contacts and through the input capacitor of the switching regulator. The magnitude of the inrush current surge depends upon the source impedance of the source of uncontrolled direct voltage, the impedance of the intervening cables, and the impedance of the input capacitor. These impedances are often not well controlled, and they may change with time. Consequently, the exact magnitude of the inrush current surge is not known, and may be large. It is often desirable to limit the magnitude of the inrush current surge which occurs at initial turn-on to prevent damage to the contacts of the input switch, to the feed cables or to other portions of the system.
U.S. Pat. No. 4,494,064 issued Jan. 15, 1985, to Harkness describes an arrangement for limiting the inrush current occurring upon connection of a capacitive load to a direct voltage with the aid of a series resistance which senses the inrush current. The sensed current is applied to a feedback loop to control a series current regulating transistor. An input inductor slows the rate of increase of the inrush current sufficiently to allow the feedback regulator to react. Such an arrangement may be undesirable for many applications because of the size and weight of an input inductor capable of handling the inrush current without saturating, and because of reliability problems associated with additional feedback circuits used solely for inrush current limiting. U.S. Pat. No. 3,781,637 issued Dec. 25, 1973, to Potter describes an arrangement in the context of an inverter for limiting inrush current into a storage capacitor by use of series current resistance element, while minimizing power dissipation of the resistance element during normal operation. In the Potter arrangement, a silicon controlled rectifier (SCR) is rendered conductive by the inverter circuit to provide a low impedance alternate current path in parallel with the resistance element. Such an arrangement continues to dissipate a significant amount of power, and may not be satisfactory where the regulator requires a voltage very near the minimum value of the uncontrolled direct voltage, because of the forward bias offset voltage of the SCR. This offset voltage is in the vicinity of one volt, and may result in significant power dissipation when the operating current requires several amperes.
U.S. Pat. No. 4,236,198 issued Nov. 25, 1980, to Ohsawa et al. describes a switching regulator with an inrush current protective resistor paralleled by the contacts of a relay which bypass the resistor during normal operation to reduce power dissipation. During normal operation, the relay in this arrangement requires continuous energization of the actuating coil, undesirably dissipating power. Furthermore, electromechanical devices such as relays may undesirably be affected by vibration and other adverse environmental conditions associated with vehicles.
An inrush current surge protective arrangement is desired which avoids the use of electromechanical devices and in which power dissipation is minimized.